JP2001311972A - Optical switch device and method for operating the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce an in-band crosstalk penalty in an add/drop device. SOLUTION: An optical switch includes a programmable light polarizing unit (POLCON) which receives an input optical signal and selects output signal polarization in response to a control signal. A polarizing beam splitter(PBS) splits the polarized signal selected from POLCON into 1st and 2nd orthogonally polarized signals. A feedback circuits couples a feedback signal presenting optical signal intensity of at least one orthogonally polarized signal with POLCON, and the POLCON uses it for adjusting the output signal polarization. The optical switch can be used with the input optical signal containing fixed or varying polarized light by using a single or two feedback signals. The optical switch is used a an optical add/drop unit, or can be included as a part of a wavelength division multiplexing(WDM) signal add/drop unit.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to wavelength division multiplexing (WD).
M) An optical switch device for use with a signal,
In particular, it relates to a programmable optical switch device configured using a controllable optical polarization unit.

[0002]

2. Description of the Related Art Programmable add / drop (add / dro)
p) The architecture of node 100 is shown in FIG. In this scheme, all WDMs in the system
Signal channel 101 is separated at demultiplexer 102. After this, some of the wavelength channels
Passing through add / drop nodes 103, other wavelength channels may be dropped or added by add / drop switch 104. Finally, all signal channels are multiplexed together by multiplexer 105 and
Form a WDM signal 106 that is sent over an LS (optical line system).

[0003] Within node 100, add / drop switch 104 may be installed one channel at a time. In this way, scaling of the number of add / drop channels can be achieved without any interruption of service to node 10
0 can be achieved. The add / drop switch is used to control the local or remote control signal 107.
May be reconfigured to change a signal channel from a drop state to a through state or vice versa. As a result, a historically reconfigurable modular switch is advantageous for such an add / drop architecture.

[0004] A potential problem with this add / drop architecture is that in-band crosstalk (in-ba
nd crosstalk). In-band crosstalk is
It is well known to cause severe performance penalties at receivers in optical networks. [EL
Goldstein, L. Eskildsen and AF Elrefaie, "Perfo
rmance implications of component crosstalk in tran
sparent lightwave networks, "IEEE Photon. Tech. Le
tt., vol.6, no.5, pp.657-660 (1994), CXYu, Wk.
Wang, and SD Brorson, "System degradation due to
multipath coherent crosstalk in WDM network node
s, "J. Lightwave Tech., vol.16, no.8, pp.1380-1386
(1998)] In-band crosstalk is referred to as an optical field that can interfere with the signal field at the receiver, resulting in a spectral beat frequency that is within the receiver bandwidth. When an optical add / drop architecture as shown in FIG. 1 is used in a network, in-band crosstalk can occur in two ways.

First, the out-of-pass band absorption constraints of the demultiplexer 102 result in multipath interference of the signal with itself at the output of the multiplexer of FIG. This contribution to the in-band crosstalk penalty increases with the number of wavelength channels, but the crosstalk level may be lower due to the second rejection at the multiplexer 105. References by Pires et al. [JJO Pires, N. Parnis, E. Jones, and M. O'Maho
ny, "Crosstalk implications in full-mesh WDMring n
etworks using arrayed-waveguide grating OADMs, "Pr
oc.ECOC'98, pp.541-542, 20-24 Sept., 1998, Madri
d, Spain] indicates that more than 35 dB of rejection is required at the demultiplexer in the full mesh WDM ring network to indicate 9 nodes.

[0006] Second, in the case of wavelength reuse, the added signal channel absorbs the imperfect drop channel (leak-through) 108 in the switch closed by 109 in FIG. By
Receive an in-band crosstalk penalty at the receiver. The leak-through field 109 of the dropped channel interferes with the added signal field 110. This is because the spectra of both the added and dropped signal channels will normally be centered.

[0007] We consider a second for different data rates.
The in-band crosstalk penalty due to the mechanism of the above is measured, and the in-band crosstalk penalty is 1 dB regardless of the granularity of the optical network.
It has been determined that the drop channel must be suppressed by 32-35 dB to ensure that it is smaller.

[0008]

Accordingly, there is a need to reduce in-band crosstalk penalties in add / drop devices.

[0009]

According to the apparatus and method of operation of the present invention, we ensure that the polarization of the signal being added is cross-polarized with respect to the linked optical field of the signal being dropped. By doing so, an optical switch that eliminates the second type of in-band crosstalk penalty is disclosed. In the case of cross polarization, there is no interference between the signal added at the receiver and the leakage through. In this case, the leak-through only contributes to the no-signal received power which results in a much smaller power penalty.

The task of cross-polarizing the added signal is complicated by the randomness of the polarization of the leak-through signal. Since the dropped signal starts at a different part of the optical network, its polarization changes over time due to a number of environmental factors. In our optical switch, we use a polarization rotor [F.
Heismann, "Analysis of a reset-free polarization
controller for fast automatic polarization stabili
zation in fiber-optic transmission systems, "J. Li
ghtwave Tech., vol. 12, no. 4 pp. 690-699], and minimizes any leak-through signals.

Thus, our optical switch offers the advantage of eliminating the in-band crosstalk power penalty resulting from interference between the optical fields of the added and leaked-down signal channels. Also, our optical switch is a modular that can be used in the wavelength add / drop node of FIG. 1 to perform the three required functions: continue-no drop, drop-and-continue, and drop / add. It is a switch that can be reconfigured over time.

More generally, our invention comprises (1) a programmable optical polarization unit for receiving an input optical signal and for selecting the polarization of an output signal in response to a control signal; (2) an OSP unit. A polarizing beam splitter (PBS) for splitting the selected polarization output signal from the first into a first and a second orthogonally polarized signal, and (3) feedback indicating the optical signal strength of the at least one orthogonally polarized signal. For an optical switch device including a feedback circuit for coupling a signal to a programmable optical polarization unit, wherein the programmable optical polarization unit adjusts its rotation in response to the feedback signal and selects Maintain a fixed polarization of the polarization output signal.

According to another aspect of the invention, the optical switch device may be included as part of an optical add / drop unit or a wavelength division multiplexing (WDM) signal add / drop unit. Optical switching devices can be used with input optical signals having fixed or varying polarization by using a single or two feedback signals, respectively. Also, the optical switch device can be embodied using various polarizing beam splitters.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the following description, the same elements in different drawings have the same reference numerals allotted to the same elements.
Also, the first digit in the code represents the figure in which the element was first shown (eg, 102 is shown first in FIG. 1). FIG. 1 shows a wavelength division multiplexing (WD)
M) shows a block diagram of a programmable add / drop node 100 for use with signals. Add / drop node 100 includes one or more add / drop switches 104, each of which receives a received WD
One or more wavelengths from the M signal 101 are dropped, dropped and continued, arted, or continued (ie, pass) under the control of the control signal 107 to form the output WDM signal 106. Programmable.

FIG. 2 illustrates a second example of an add / drop switch device 200 that may be used with the add / drop node 100 of FIG. 1 to control add / drop / continue capabilities for one or more wavelengths. 1 shows an embodiment. Add / Drop Switch Apparatus 200
Receives an input optical wavelength signal 201 (e.g., from the demultiplexer 102) and rotates the polarization of the optical field of the input signal 201 at the output 201a to a desired polarization.
LCON) 202.

As shown, the polarization 230 of the light field of the input signal 201 may be of any type and may vary as a function of time. As the input signal occurs in different parts of the optical network, its polarization changes over time due to a number of environmental factors. Various polarizations of the input signal 201 may be caused, for example, by a variety of environmental factors that affect the fiber optic facility over which the input signal was carried. These environmental factors may include the temperature, pressure, interference, etc., to which the fiber optic facility is exposed. Generally, the polarization of the input signal 201 changes at a rate of 10 to 100 μs. However, such a rate of polarization change can be easily compensated for by our feedback configuration used with POLCON 202.

The light output signal 201a of the POLCON 202
Is the first fiber polarization beam splitter (PBS1)
As indicated by 成分 and ◎ (corresponding to the crosses in the circles in the figure), the component is divided into orthogonally polarized light components 203. Polarization state at each of the two outputs of PBS1,
That is, Δ and ◎ indicate the polarization maintaining (PM) fiber 205.
And 204 are retained. The 10% couplers or taps 206 and 207 on each output path are terminated at photodetectors 208 and 209, respectively.

Two photodetectors, PD @ 209 and PD
２０８208 monitors the power level of each of the two polarization states in the output signal of PBS1 203. The electric photocurrent signals 210 and 211 from the photodetectors 208 and 209 are input to the control circuit (401 in FIG. 4) of the POLCON 202, respectively. Photocurrent signal I _{PD ↑} 21
The ratio of 1 and I _{PD ◎} 210 changes the polarization state of input signal 201a to PBS1 regardless of the total power in the signal channel.
Give to 203.

Therefore, when the ratio of photocurrent signal 211 to 212 is low, the input signal to PBS1 203 has more ◎ polarization than 偏光 polarization. Conversely, if the ratio of photocurrent signal 211 to 210 is high, then PBS12
The input signal to 03 has more ↑ polarization than ◎ polarization. Two feedback loops are formed in FIG. 2, one is formed by PBS1, coupler 206, PD208 and POLCON202, the second is PBS1, coupler 207, PD207 and POL
It is formed by CON 202.

A control signal 220 selects whether the POLCON 202 should be in drop (and add), continue, and drop and continue modes, and outputs a polarization signal of ◎, ↑, or ◎ and か ら from the POLCON 202, respectively. Let it. Feedback signals 210 and 211 are
Adjusts its polarization rotation to compensate for polarization variations (eg, 230) in the input signal 201, and therefore POLC
It enables the selected polarization of the output signal 201a of the ON 202 to be kept constant.

When the control signal 220 is the input optical signal 201,
POLCO to indicate that it should be dropped
In N202, PBS1 leads to drop port 221.
Generate a polarization signal. The control signal 220 is an optical signal 220
Should be passed through the add / drop switch 200 (or continue).
2 generates a ↑ -polarized signal that the PBS 1 directs to the fiber 205. Then, the PBS1 203 provides the drop function to the add / drop switch device 200.

The add function is provided by the add port 223.
Provided by PBS2 222 receiving the polarization signal;
The polarization signal is combined on the fiber 205 with the ↑ polarization signal to produce an output signal 224. For example, control signal 2
If 20 selects that POLCON 202 should be in the drop and continue mode (here, if both ↑ and 偏光 choose to be present in signal 201a), then the ↑ polarized signal is present.

The output signal 224 of the PBS 2 is multiplexed with another signal channel by the multiplexer 105 to form the WDM signal 106. The polarizing beam splitters PBS1 and PBS2 described above may be of a type selected from the group comprising prisms, thin glass plates, partially reflective mirrors, bulk optical devices or other known types.

Illustratively, any two fixed orthogonal linear polarizations, represented by ↑ and ◎, are POLCON20
It should be noted that the signals from the two can be set as two output polarization states. The continuation (or through) mode or state of the switch 200 is assigned a polarization state of ↑, and the drop / add mode is assigned a polarization state of ◎.

FIG. 4 shows a block diagram of the POLCON 202 used in the add / drop device of FIG. As shown, the POLCON 202 includes a control circuit 4
01 and the polarization rotator 404. Polarization rotor 4
04 may be a polarization transformer of the type shown in U.S. Pat. No. 5,930,414 issued Jul. 27, 1999 to DA Fishman et al. Control circuit 40
1 includes an operational amplifier 402 that determines the ratio of the two photocurrents 210 and 211.

The output of the amplifier 402 is supplied to the mode selection circuit 403 together with the control signal 220. Control signal 22
0 indicates that the mode selection circuit 403 determines that the control polarization rotator 4
04 enables the output of the appropriate voltage needed to select the operating mode. Depending on the particular mode of operation selected, control polarization rotator 404 sets the linear state of polarization of its output signal 201a. Then, the control signal 220 is transmitted to the add / drop switch 20.
0 allows each of the functions to be performed as follows.

Continue-no-drop mode: This is obtained by compensating that the feedback sets the output state of POLCON 202 to the ↑ -polarization state. In other words, the ratio I _{PD ↑} / I _{PD ◎} is set to a fixed large value (eg, 27 dB). Therefore, since the polarization of the input signal changes with time, the feedback signal I _{PD ↑} / I _{PD ◎} causes the polarization rotator 404 to change its rotation so as to maintain the polarization of the output signal 201a of the POLCON 202 in the ↑ polarization state. . The upper limit of this ratio is set by the polarization absorption of PBS.

Drop and Continue Mode: Depending on the required absorption between the drop / through state of the add / drop, the ratio I _{PD ド ロ ッ プ} / I _PDＩ can be set. The ratio of dropped signal to continued signal is determined by the particular application. In a multicast application, the drop signal level may be determined by the number of nodes that will receive the same optical signal.

Drop and add mode: this is the ratio I
It is obtained by setting _{PD ↑} / I _{PD PD} to a fixed low value (eg, -27 dB). This is naturally cross-polarized in the through (◎) polarization state, since the polarization of the added signal can be fixed and set to the polarization state. Since the polarization of the input signal changes with time, the feedback signal I _{PD ↑} / I _{PD ◎}
The rotation of the polarization rotator 404 is changed so that the polarization of the output signal 201a is maintained in the ◎ polarization state. ◎
The leak-through signal in the polarization state is rejected twice in PBS2. Therefore, the total rejection of the drop channel is the ratio of I _{PD ↑} / I _{PD ◎}
It is twice.

[0030] Our add / drop switch 200
It has two important advantages. First, the add / drop switch 200 can be a separate module that can be installed on a particular signaling channel at the add / drop node 100 without interrupting service on other channels. Second, add / drop switch 2
00 requires a control signal 220 that specifies only one number, the ratio I _{PD ↑} / I _{PD ◎} , and is remotely reconfigurable (rec
onfigurable). Control signal 220 may be provided by a node computer or an optical network control computer.

Also, our add / drop optical switch ensures that the polarization of the added signal is cross-polarized with respect to the link-through optical field of the dropped signal. Because of this cross polarization, the leak-through signal does not interfere with the added signal when the added signal is detected at the receiver. In this case,
The leak-through signal contributes only the ratio signal reception power, which leads to a much smaller power penalty. Since our optical switch 200 provides a feedback signal that controls the polarization rotator 404 to maintain the drop channel signal in a fixed state of linear polarization, the optical switch 20
Minimize the leak-through signal passing through zero.

Therefore, our optical switch 200 is
It offers the advantage of eliminating the in-band crosstalk power penalty resulting from interference between the added signal channel and the optical field of the leak-through-drop signal channel. Also, our optical switch 200 is a modular, historically reconfigurable switch that has three required functions: continuation drop,
It can be used at the wavelength add / drop node of FIG. 1 to perform drop and continue and drop / add.

FIG. 3 shows the architecture of an alternative add / drop switch 200, where:
The single bulk optical PBS1 301 performs both the add function and the drop function. But this is PB
S1 301 imposes severe restrictions on the polarization rejection ratio. If this ratio is less than 35 dB, the dropped signal will suffer an in-band crosstalk penalty due to the added signal channel.

FIG. 5 illustrates one embodiment of a drop / continue switch device used with a fixed polarization input signal 501. Since the polarization of the input signal 501 is fixed, the PBS 503, the coupler 506, the PD 508, and the P
The feedback circuit including the OLCON 502 is connected only at a predetermined time (for example, at the time of startup).
In the setup, the rotation of the polarization rotator of POLCON 502 is adjusted to obtain the desired polarization of output signal 501a. That is, the ↑ polarization state for the continue mode, the ◎ polarization state for the drop (and add) mode, and the 偏光 and 偏光 polarization states for the drop and continue mode.

[0035]

As described above, according to the present invention,
In-band crosstalk penalties can be reduced in add / drop devices.

In the claims, the reference numerals in parentheses after the constituent features of the invention are set forth in order to facilitate understanding of the invention in association with the constituent features and the embodiments. It should not be used to interpret

[Brief description of the drawings]

FIG. 1 is an exemplary wavelength division multiplexing (WDM) according to the present invention.
FIG. 4 is a block diagram illustrating a programmable add / drop node for use with signals.

FIG. 2 illustrates a first embodiment of an add / drop optical switch that may be used in the add / drop node of FIG. 1 to control add / drop / continue capabilities for one wavelength.

FIG. 3 illustrates a second embodiment of an add / drop optical switch that may be used in the add / drop node of FIG. 1 to control add / drop / continue capabilities for one wavelength.

FIG. 4 is a block diagram illustrating an electro-optical polarization controller (POLCON) used in the add / drop device of FIGS. 2 and 3;

FIG. 5 is a diagram showing one embodiment of a drop / continue optical switch device.

[Explanation of symbols]

 REFERENCE SIGNS LIST 100 programmable add / drop node 101 WDM signal channel 102 demultiplexer 104 add / drop switch 105 multiplexer 106 WDM signal 107 control signal 200 add / drop switch device 201 input optical wavelength signal 203, 222 polarization beam splitter 204, 205 polarization maintaining fiber 206 , 207 Coupler 208, 209 Photodetector 210, 211 Electric photocurrent signal 220 Control signal 221 Drop (DROP) 223 Add (ADD) 224 Output signal

 ──────────────────────────────────────────────────続 き Continuation of the front page (71) Applicant 596077259 600 Mountain Avenue, Murray Hill, New Jersey 07974-0636 U.S.A. S. A. (72) Inventor Luc Boyban United States, 07724 New Jersey, Eatontown, White Street 58B (72) Inventor Zitan Saracy United States, 07724 New Jersey, Eatontown, Country Club Road, Apartment 79

Claims (15)

[Claims] 1. A programmable optical polarization unit for receiving an input optical signal and selecting a polarization of an output signal in response to a control signal, and a selected polarization output signal from the programmable optical polarization unit. A polarizing beam splitter (PBS) for splitting into first and second orthogonally polarized signals; and a feedback signal indicative of at least one optical signal strength of the orthogonally polarized signals to the programmable optical polarization unit. A feedback circuit, wherein the programmable optical polarization unit adjusts the polarization of the selected polarization output signal in response to the feedback signal. 2. In response to a first control signal, the programmable optical polarization unit selects the first orthogonally polarized signal, and in response to a second control signal, the programmable optical polarization unit. 2. The optical switch device according to claim 1, wherein the selector selects the second orthogonally polarized signal. 3. In response to a third control signal, the programmable optical polarization unit selects a polarized signal that includes a portion of both the first and second orthogonally polarized signals. The optical switch device according to claim 2, wherein 4. The polarization of the input optical signal remains substantially constant, and the feedback circuit couples only one of the first and second orthogonally polarized signals to the programmable optical polarization unit. 2. The optical switch device according to claim 1, wherein 5. The optical switch device according to claim 4, wherein the feedback circuit is enabled only during a predetermined condition. 6. The polarization of the input optical signal changes with time, and the feedback circuit includes the first and second input signals.
Are coupled to the programmable light polarization unit, the programmable light polarization unit adjusting its polarization rotation to maintain the selected polarization output signal at a fixed polarization. The optical switch device according to claim 1, wherein 7. The feedback circuit includes: a coupler for coupling a part of the first orthogonally polarized signal to a first photodetector, wherein the photodetector includes the first quadrature polarized signal. Responsive to the combined portion of the polarized signal to generate an electrical signal, wherein the programmable optical polarization unit adjusts the polarization of the selected polarization output signal in response to the electrical signal. The optical switch device according to claim 1, wherein: 8. The feedback circuit includes a first feedback loop and a second feedback loop, wherein the first feedback loop detects a portion of the first orthogonally polarized signal with a first light detection signal. A first coupler for coupling to the detector, wherein the first photodetector generates a first electrical signal in response to the combined portion of the first orthogonally polarized signal. The second feedback loop includes a second coupler for coupling a portion of the second orthogonally polarized signal to a second photodetector, wherein the second photodetector comprises: Responsive to the combined portion of the second orthogonally polarized signal to generate a second electrical signal, wherein the programmable optical polarization unit is responsive to both the first and second electrical signals And the selected polarization 2. The method according to claim 1, wherein the polarization of the output signal is adjusted.
An optical switch device according to claim 1. 9. A control unit for generating a polarization adjustment signal in response to the feedback signal and the control signal, the programmable light polarization unit; and selecting the polarization in response to an adjustment signal. The optical switch device according to claim 1, further comprising a polarization rotator for adjusting the polarization of the polarized output signal. 10. The polarization beam splitter, configured as an add / drop unit, wherein the polarization beam splitter operates as a drop unit, wherein a first orthogonally polarized signal is output at a first port and is coupled to the second orthogonally polarized signal. The optical switch device receives the first orthogonally polarized signal at a first port, and receives an add second orthogonally polarized signal at an add port. The optical switch device according to claim 1, further comprising a second polarizing beam splitter (PBS) connected to generate a combined output signal at an output port. 11. The polarization beam splitter and the second polarization beam splitter.
The optical switching device of claim 1, wherein the polarizing beam splitter is selected from the group consisting of a prism, a thin glass plate, a partially reflective mirror, and a bulk optical device. 12. The optical polarization beam splitter, configured as an add / drop unit, wherein the optical polarization beam splitter is a bulk optics device including four ports and providing an add / drop function, wherein the first orthogonally polarized signal is: The second orthogonally polarized signal output at the first port is:
A second orthogonally polarized signal output at the drop port and added is received at the add port and the bulk optics device converts the first orthogonally polarized signal to the added second orthogonally polarized signal. The optical switch device according to claim 1, wherein the optical switch device combines the output signal and the output signal at a fourth port. 13. A demultiplexer configured as a wavelength division multiplexing (WDM) signal add / drop unit for separating an input WDM signal into a plurality of output wavelength channels, and a plurality of input wavelength channels into one output WDM signal. A multiplexer for multiplexing; an optical connection path for coupling an output wavelength channel of the demultiplexer to an input wavelength channel of the multiplexer; and at least one add / drop unit, each add / drop unit comprising: Connected in series with different optical connection paths between the output wavelength channel and the input wavelength channel, wherein the drop port of the first PBS is responsive to a separate control signal being received at the add / drop unit. , Used to drop the wavelength channel of the input WDM signal; Serial add port of the second PBS is an optical switch device according to claim 10, characterized in that it is used to add a wavelength channel to the output WDM signal. 14. The optical switch device according to claim 1, wherein the polarizing beam splitter is selected from a group including a prism, a thin glass plate, a partially reflective mirror, and a bulk optic device. 15. A method for receiving an input optical signal at a programmable optical polarization unit and selecting a polarization of an output signal in response to a control signal, the method comprising: converting the selected polarization output signal to first and second orthogonal polarizations. Splitting the optical signal into at least one orthogonally polarized signal; coupling a feedback signal indicative of an optical signal strength of at least one orthogonally polarized signal to the programmable optical polarization unit; and responsive to the feedback, the programmable optical polarization unit Adjusting the polarization of the selected polarization output signal in the method.